You can believe that something is both heritable and flexible. Most personality traits are at maximum about 50% heritable; I don't think that's so bad.
Height is heritable yet highly subject to manipulation at a young age.
You are right in not buying into that.
Henry Ford:
If you think you can do a thing or think you can't do a thing, you're right.
"Those who say it can't be done are usually interrupted by others doing it."
-James A. Baldwin